Integrated circuit device having improved post for surface-mount package

ABSTRACT

An electronic device (10) includes a package (16) having two posts (30) suitable for insertion in PCB holes. Package (16) presents a lengthwise molding plane (32) along which the upper portion (42) and bottom portion (44) of package (16) are mated during the molding process. Posts (30) are disposed substantially exclusively in bottom portion (44) so that posts (30) are asymmetric about lengthwise molding plane (32). Thus, even if a top mold (42a) and a bottom mold (44a) are misaligned there will be no effect on the dimensional tolerance of posts (30) and thus the tolerance of post (30) can be closely matched with a PCB hole (20) tolerance to insure a snug fit. Thus, device (10) is mounted edgewise on a PCB (18) by insertion of posts (30) into PCB holes (20) so that tips (24) of lead fingers (4 14) can be connected to PCB (18) by surfacing-mounting techniques or the like.

This is a division of application Ser. No. 07/723,108, filed Jun. 28,1991, now U.S. Pat. No. 5,204,287.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices, and more particularlyto surface-mount packages for integrated circuit devices.

2. Description of the Prior Art

Integrated circuit devices have been housed in dual-in-line plasticpackages, referred to as DIP packages, in the bulk of semiconductormanufacture, for many years. These DIP packages have leads which extendthrough holes in a printed circuit board (PCB), and the package itselfis mounted flat on the board. More recently surface-mount packaging hasbeen introduced, and this technique eliminates the necessity ofsoldering leads in holes in PC boards, so the leads and PC boardconductors can be closer together and higher densities are achieved.Similarly, edge-mounted packages or modules have been used in an effortto further increase the density of devices, improve cooling and reducecost. As the density of devices and circuitry on the dies continues toincrease, the packing density must also increase, both at the packagelevel and at the board level, while at the same time providing adequatecooling, and allowing the assembly and soldering of packages and boardsto be fast, accurate and nondestructive. In particular, the packagingtechnique must provide protection for the leads during test, burn-in,and transport, as well as ease of positioning and mechanical supportduring mounting and soldering and during the life of the system.

SUMMARY OF THE INVENTION

The problems outlined above are in large measure solved by the method inaccordance with the present invention. That is to say, the inventionhereof provides a device which can be mechanically positioned on a PCBin a stable, secure fashion so that surface-mount or other processes canbe performed. The process provides a method wherein the tolerancesbetween the posts of the package and the holes of the PC board may beclosely matched. The device and associated method in accordance with thepresent invention include a package encapsulating a die with a pluralityof lead fingers extending from at least one edge of the package. Inpreferred embodiments, two posts extend from the package from the sameedge so that the package may be vertically mounted on the PCB board in alengthwise fashion. When the device is manufactured the posts are formedin only one of the molds so that tolerances between the post and the PCBholes may be closely matched.

The invention provides an integrated circuit device comprising:

a package encapsulating a semiconductor die, said package presenting alengthwise molding plane;

a plurality of lead fingers extending from at least one edge of saidpackage, said lead fingers electrically connected to said die; and

a plurality of posts integral with said package and extending from saidat least one edge of said package, said posts asymmetrically shapedabout said lengthwise molding plane to fit into holes in a printedcircuit board for mechanically positioning and supporting said packagewhen said lead fingers are affixed to the printed circuit board.

One object of this invention is to provide an improved packaging methodfor semiconductor integrated circuit devices. Another object is toprovide an edge-mounted package for semiconductor devices that does notrequire soldering of leads into through-holes in a printed circuitboard. A further object is to provide a surface-mount package forintegrated circuits that is mechanically secure during mounting,soldering and operation, yet does not occupy excess space on the PCboard. Still another object is to provide an edge-mount, surface-mountmethod for integrated circuit devices that allows the leads to beprotected during the time from assembly to final system use, and allowsaccurate positioning of the leads on a PC board. Yet another object isto provide a package with posts such that the dimension tolerances ofthe posts can be closely matched with tolerances of the PCB holes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a depiction of an idealized post of a prior art device;

FIG. 1B is a depiction of a practical post of a device of the prior art;

FIG. 2 is an elevational view of an electronic device incorporating theinvention, wherein the device is surface-mounted on a printed circuitboard;

FIG. 3 is a bottom view of the electronic device of FIG. 2 as viewedfrom the direction of arrows 3;

FIG. 4 is a sectional view of device 10 taken along line 4--4 of FIG. 2;

FIGS. 5A-5C are progressive sectional views taken at line 5--5 of FIG. 2illustrating progressive steps in the insertion of a post of the deviceinto a hole on the PCB;

FIG. 6 is a partial sectional view analogous to FIG. 4, of a moldsurrounding the device;

FIG. 7 is a partial elevational view of a system incorporating aplurality of inventive devices surface-mounted on a PCB.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings in general and FIG. 1A in particular, aprior art idealized post 130 is shown inserted into a hole 120 of aprinted circuit board or PCB (not shown) so that the complete priordevice (not shown) can be vertically mounted on the PCB. FIG. 1A showsan idealized post 130 having a top post portion 122 and a bottom postportion 124 which are perfectly mated and aligned. By way of example,edge 126 has a length of 1.016-mm. The effective diameter for post 130is about 1.3572-mm, and is indicated by the arrows spanning hole 120 inFIG. 1A.

FIG. 1B shows an actual prior art post 230 inserted into a hole 220.Post 230 exhibits molding misalignment between top post portion 122 andbottom post portion 124. This misalignment results in an effectivediameter for post 230 of about 1.3931-mm, indicated by the arrowsspanning hole 220 in FIG. 1B.

The effective diameter of post 230 is about 0.0359-mm greater than theeffective diameter of post 130. (It should be noted that FIGS. 1A and1B, as well as the rest of the drawings are not drawn to scale.) Thus,in the prior art, the misalignment of post portions 122, 124 results ina greater tolerance required for practical PCB holes 220. There is agreater likelihood that post 230 will not fit snugly into the hole. Inthis fashion, the prior art device is unstably positioned for surfacemounting which can lead to poor mechanical positioning of the leads,poor reflow solder connections, etc. What is needed is a post withoutsuch potential misalignment of the top and bottom portions so that theactual tolerance of the post and hole can be closely correlatedresulting in a snug fit.

Turning now to the present invention and referring to FIG. 2, anelectronic device 10 has a die 12 (see FIG. 4), a plurality of leadfingers 14, and a package 16. Package 16 is adapted to be fitted into aprinted circuit board (PCB) 18 having a plurality of holes 20 so thatdevice 10 is stably, securely mounted on PCB 18 both before and aftersolder reflow. PCB 18 can be any kind of a circuit board, hybridized orotherwise.

Referring to FIG. 4, device 10 is shown cross sectionally in a viewtaken along line 4--4 as shown in FIG. 2. Die 12 is shown encapsulatedby package 16 in FIG. 4. Die 12 is conventional in nature. Die 12includes the typical components of the microelectronic art and isgenerally made of monocrystalline silicon but other substrate materialscould also be used--for example, but not by way of limitation, germaniumand gallium arsenide. In one embodiment, die 12 is a 4 megabit DRAM diesuch as the type manufactured by Texas Instruments Incorporated ofDallas, Tex., but other types of dies well known in the microelectronicindustry (for example any other type of memory device, logic device orindeed discrete devices and other kinds of integrated devices), couldutilize the present invention and are contemplated as being within therealm of the invention.

Still referring to FIG. 4, lead fingers 14 are conventionally attachedto die 12 at bonding pads or the like. Each lead finger 14 has a leadfoot 22 extending from package 16. Each foot 22 includes a tip 24 whichis surface-mounted to PCB 18 in a conventional fashion (see also FIG.2). It should be noted that PCB 18 is omitted from FIG. 4 for clarity ofillustration.

Referring again to FIG. 2, package 16 includes package body 26, poststops 28 and two posts 30 (posts 30 are shown in phantom in FIG. 2).Body 26 presents a lengthwise molding plane which is in the plane of thepage as viewed in FIG. 2 (reference numerals 32 in FIG. 3 also indicatethe lengthwise plane). Body 26 is conventionally formed by, for example,molding to envelope die 12. Posts 30 are adapted to snugly fit into PCBholes 20.

Referring to FIG. 3, device 10 is shown from a bottom view as indicatedby arrows 3 in FIG. 2. PCB 18 is omitted for clarity of illustration.Posts 30 are more clearly seen to be tapered in FIG. 3. It will also benoted that posts 30 are asymmetrically shaped about the lengthwisemolding plane indicated by reference numerals 32, which show an axialline cutting device 10 in a plane transverse to the plane of the page asviewed in FIG. 3. The lengthwise plane indicated by reference numerals32 is the mating plane for molding purposes (see FIG. 6). That is tosay, posts 30 are disposed substantially exclusively above thelengthwise plane while substantially no portions of posts 30 aredisposed below the lengthwise plane.

Referring now to FIGS. 5A-5C, progressive, detailed sectional views areshown of one post 30 situated in one hole 20 taken from line 5--5 asshown in FIG. 2. All other portions of device 10 and PCB 18 have beenomitted from FIGS. 5A-5C except those portions shown, for clarity ofillustration. In addition to showing the perimeter of PCB hole 20, FIGS.5A-5C each utilize arrows to indicate an effective diameter of post 30at the hole 20. This is done for comparison of the effective diameter ofpost 30 with the diameter of hole 20 as post 30 is progressivelyinserted. In the preferred embodiment the diameter of PCB hole 20 is1.194-mm (roughly 47 mils).

FIG. 5A shows a view of post 30 at the beginning of insertion into PCBhole 20. Post 30 has an effective diameter of 1.096-mm. This effectivediameter describes a circle indicated by reference numeral 34. FIG. 5Bshows an intermediate stage of insertion of post 30 into hole 20. Theeffective diameter of post 30 at this stage of insertion is about1.160-mm. The perimeter of the circle described by this effectivediameter is indicated by reference numeral 36 and is substantially thesame as the perimeter of hole 20 itself as indicated in FIG. 5B. FIG. 5Cshows the final stage of insertion where post 30 has an effectivediameter of about 1.270-mm. The perimeter of the circle described bythis effective diameter is indicated by reference numeral 38. Thiseffective diameter will result in interference with holes 20 on PCB 18to ensure snug fit for processing.

FIG. 6 shows a sectional elevational view of device 10 enveloped by aninventive mold set 40. It will be noted that device 10 is shown in areverse orientation compared with that of FIG. 4. PCB 18 is not shownbecause the molding process takes place prior to mounting of device 10on PCB 18. It is further seen that post 30 is asymmetrical about thelengthwise plane (which is transverse to the plane of the page as viewedin FIG. 6), the lengthwise plane once again being indicated by referencenumerals 32.

Mold set 40 includes a top mold 42a which forms top portion 42 ofpackage 16, and a bottom mold 44a which forms a bottom portion 44 ofpackage 16 (see also FIG. 3). Top and bottom molds 42a, 44a mate at thelengthwise plane. Before molding, die 12 is mounted and then wire bondedto the leadframe having lead fingers 14. Device 10 is then placed inbetween molds 42a, 44a for encapsulation. It will be readily appreciatedthat lead fingers 14, as shown in FIG. 6, are not formed to the finalorientation as shown in FIG. 2.

FIG. 7 shows an electronic system 46 having a plurality of devices 10mounted on PCB 18 in accordance with the invention. Devices 10 arevertically mounted on PCB 18 to maximize packing density and furthertake advantage of the beneficial aspects of the invention as describedherein.

The process for manufacturing device 10 and/or system 46 will now bedescribed. Die 12 (with lead fingers 14 conventionally mounted thereon)is placed in top mold 42a. Top and bottom molds 42a, 44a are mated at amold juncture along the lengthwise plane as indicated from differentviews in FIGS. 3 and 6. A mold compound, for example a synthetic resinmaterial such as Shinetsu 165 vA is then injected into mold set 40 andcured. Molds 42a, 44a are then removed and lead fingers 14 are formed,yielding device 10 as shown in FIG. 4. A plurality of holes 20 are thendrilled in PC board 18. In the preferred embodiment, each device 10 willhave two posts 30, but any other number of posts can be used dependingon the application. Posts 30 of a plurality of devices 10 are theninserted into corresponding PCB holes 20. The tapered profile of posts30 facilitates this insertion. Devices 10 are then reflow soldered onPCB 18 by a conventional process such as Infra-red reflow or vapor phasereflow at lead tips 24, such processing being well known in the art.Thus system 46 is completed.

It will be readily appreciated that a system could have only one device10 rather than a plurality of devices 10, and the single device 10 couldbe surface-mounted on PCB 18 as described above, but in the preferredembodiment a plurality of devices 10 is mounted, such as shown in FIG.7.

Since the final effective diameter after insertion (as described bycircle 38 in FIG. 5C) is greater than the diameter of PCB hole 20, asnug fit is guaranteed. Stops 28 insure that tapered posts 30 are notover-inserted, thereby avoiding inadvertent damage to lead fingers 14.Thus an effective surface-mounting process is facilitated.

It will be noted that posts 30 are formed only in bottom mold 44a.(Those skilled in the art will readily appreciate that the distinctionbetween top mold 42a and bottom mold 44a is relative and the terms topmold and bottom mold are essentially interchangeable.) Since posts 30are formed only in bottom mold 44a, they are thus formed asymmetrically.See FIG. 3 where it is demonstrated that posts 30 reside substantiallyexclusively above the lengthwise plane indicated by reference numerals32 and substantially no portion of either post 30 resides belowlengthwise plane 32. In the illustrated embodiment, the posts 30 areformed at the juncture of molds 42a, 44a, with a top surface coincidentwith the lengthwise mold plane and a tapered tip. The tip has agenerally trapezoidal cross-section, with base defined by the topsurface and a top and sides defined by other post surfaces havingtapered tip ends formed in bottom mold 44a. Since the potential fordetrimental misalignment (as illustrated in FIG. 1B) is eliminated, thediameter of post 30 can be closely matched with the diameter of PCB hole20. Hence PCB 18 can have tighter tolerances for holes 20, insuringtight fit of posts 30 into holes 20. Devices 10 will thus be securelyfitted on PCB 18 allowing effective surface-mounting procedures. Thus,according to the invention, the improved posts 30 allow for a snuggerfit of posts 30 into holes 20 for greater mechanical stability and abetter resultant surface-mounting procedure.

Although the invention has been described in detail herein withreference to its preferred embodiment and certain describedalternatives, it is to be understood that this description is by way ofexample only, and is not to be construed in a limiting sense. It is tobe further understood that numerous changes in the details of theembodiments of the invention, and additional embodiments of theinvention, will be apparent to, and may be made by, persons of ordinaryskill in the art having reference to this description. It iscontemplated that all such changes and additional embodiments are withinthe spirit and true scope of the invention as claimed below.

What is claimed is:
 1. An integrated circuit device having an improvedpost for surface-mount packaging, said device comprising:an integrallymolded package encapsulating a semiconductor die, said packagepresenting a lengthwise molding plane; a plurality of lead fingersextending from at least one edge of said package, said lead fingerselectrically connected to said die; and a plurality of posts integralwith said package and extending from said at least one edge of saidpackage, said posts asymmetrically shaped about said lengthwise moldingplane to fit into holes in a printed circuit board for mechanicallypositioning and supporting said package when said lead fingers areaffixed to the printed circuit board.
 2. The device of claim 1 whereinat least one of said posts is formed on one side of said lengthwisemolding plane.
 3. The device of claim 1 wherein said packagesubstantially includes synthetic resin material molded around said dieand bonded lead fingers.
 4. The device of claim 1 wherein said leadfingers are bent to present a face perpendicular to said lengthwisemolding plane.
 5. An electronic circuit board system comprising:aprinted circuit board having a plurality of holes; and a plurality ofintegrated circuit devices mounted on said board, at least one of saiddevices edge-mounted on said board; said at least one device including:apackage molded around a semiconductor die, said package presenting alengthwise molding plane; a plurality of lead fingers extending from atleast one edge of said package, said lead fingers electrically connectedto said die; and a plurality of posts integral with said packageextending from said at least one edge of said package, at least one ofsaid posts asymmetrically shaped about said lengthwise molding plane tofit into one of said holes in said printed circuit board formechanically positioning and supporting said package when said leadfingers are affixed to said printed circuit board.
 6. The system ofclaim 5 wherein said at least one post is formed on one side of saidlengthwise molding plane.
 7. The system of claim 5 wherein said devicesare edge-mounted devices parallel to one another, and each device has apackage encapsulating a semiconductor die, said package presenting alengthwise molding plane, and each device further includes at least onepost asymmetrically shaped about said lengthwise molding plane.
 8. Asemiconductor packaged device, comprising:a semiconductor dieencapsulated in a molded package formed of two integral portions matedat a lengthwise mold juncture; a plurality of lead fingers, at leastsome of which extend from an edge of said package and are electricallyconnected to said die; and at least one post extending from said edgebeyond said at least some lead fingers and from only one of the integralportions, integral therewith; said at least one post being dimensionedand configured relative to said lead fingers so that said at least onepost can be fitted into a hole in a printed circuit board formechanically positioning and supporting said package while said leadfingers are electrically connected to a facing surface of said board. 9.The device of claim 8 wherein said mold juncture is a lengthwise moldingplane and said at least one post has a surface coincident with saidlengthwise molding plane.
 10. The device of claim 9 wherein said posthas a tip with generally trapezoidal cross-section and three tapered tipsurfaces, with said coincident surface forming a base and said taperedtip surfaces respectively forming a top and sides of said trapezoidalcross-section.
 11. The device of claim 10, in combination with saidprinted circuit board and hole, said hole having a diameter, and saidtapered surfaces being dimensioned and configured so that when said atleast one post is fitted within said hole, said trapezoidalcross-section has an effective diameter describing a circle whoseperimeter is less than the perimeter of said hole diameter at abeginning of insertion, substantially the same as said hole perimeter atan intermediate stage of insertion, and in interference with said holeperimeter at a final stage of insertion.
 12. The device of claim 10wherein said device has two posts having the recited characteristics ofsaid at least one post.
 13. The device of claim 8 wherein said post hasa tip with generally trapezoidal cross-section and three tapered tipsurfaces, with said coincident surface forming a base and said taperedtip surfaces respectively forming a top and sides of said trapezoidalcross-section.
 14. The device of claim 13, in combination with saidprinted circuit board and hole, said hole having a diameter, and saidtapered surfaces being dimensioned and configured so that when said atleast one post is fitted within said hole, said trapezoidalcross-section has an effective diameter describing a circle whoseperimeter is less than the perimeter of said hole diameter at abeginning of insertion, substantially the same as said hole perimeter atan intermediate stage of insertion, and in interference with said holeperimeter at a final stage of insertion.
 15. An integrated circuitdevice, comprising:a package encapsulating a semiconductor die, saidpackage including a plurality of posts integral therewith and extendingfrom at least one edge of said package, said package presenting alengthwise molding plane; and a plurality of lead fingers electricallyconnected to said die, said lead fingers extending from at least oneedge of said package; at least one of said posts being only on one sideof said lengthwise molding plane so as to fit into holes in a printedcircuit board for mechanically positioning and supporting said packagewhen said lead fingers are affixed to a printed circuit board.